Photovoltaic ultraviolet sensor

ABSTRACT

A photovoltaic ultraviolet sensor comprises a zinc oxide single crystal substrate. On the +c face of the zinc oxide single crystal substrate, an ultraviolet receiver is formed. The exemplary ultraviolet receiver includes a Schottky electrode which, when receiving ultraviolet rays, produces a voltage in cooperation with the zinc oxide single crystal substrate. The ultraviolet sensor does not have any sensitivity to the visible rays. The ultraviolet sensor has a relatively fast response of several microseconds.

BACKGROUND OF THE INVENTION

This invention relates to a photovoltaic ultraviolet sensor comprising azinc oxide single crystal.

JP-A H03-241777, which is incorporated herein by reference in itsentirety, discloses a photoconductive ultraviolet sensor which comprisesa substrate and a zinc oxide thin film formed on the substrate. In JP-AH03-241777, the zinc oxide thin film preferably has an optical forbiddenband of about 3.0 to about 3.2 eV. However, the zinc oxide thin film ofthe foregoing optical forbidden band is sensitive also to visible raysand therefore has insufficient sensitivity to ultraviolet rays.

JP-A H10-182290, which is incorporated herein by reference in itsentirety, discloses another ultraviolet sensor which comprises a zincoxide crystal whose “a” face is used as an ultraviolet receiver surface.To sense ultraviolet rays, impedance variation of the zinc oxide crystalis monitored while the zinc oxide crystal is supplied with electricfields according to an antiresonance frequency of the zinc oxidecrystal. However, generation of the antiresonance frequency requiresspecial equipment so that the total cost of the ultraviolet sensorbecomes high. In addition, the ultraviolet sensor of JP-A H10-182290 hasa relatively slow response because of its sensing mechanism.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an ultravioletsensor which is not sensitive to visible rays and has a relatively fastresponse without using complex equipment.

According to one aspect of the present invention, a photovoltaicultraviolet sensor comprises a zinc oxide single crystal and anultraviolet receiver. The zinc oxide single crystal has a +c face. Theultraviolet receiver is formed on the +c face of the zinc oxide singlecrystal and, when receiving ultraviolet rays, produces a voltage solelyor in cooperation with the zinc oxide single crystal.

An appreciation of the objectives of the present invention and a morecomplete understanding of its structure may be had by studying thefollowing description of the preferred embodiment and by referring tothe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partially-sectional, perspective view showing a photovoltaicultraviolet sensor according to a first embodiment of the presentinvention;

FIG. 2 is a generally schematic view showing a zinc oxide single crystalblock;

FIG. 3 is a generally schematic view showing a crystal structure of azinc oxide single crystal;

FIG. 4 is a sectional view showing a fabrication process for thephotovoltaic ultraviolet sensor of FIG. 1;

FIG. 5 is a sectional view showing a subsequent fabrication process forthe photovoltaic ultraviolet sensor of FIG. 1;

FIG. 6 is a sectional view showing a subsequent fabrication process forthe photovoltaic ultraviolet sensor of FIG. 1;

FIG. 7 is a sectional view showing a subsequent fabrication process forthe photovoltaic ultraviolet sensor of FIG. 1;

FIG. 8 is a sectional view showing a subsequent fabrication process forthe photovoltaic ultraviolet sensor of FIG. 1;

FIG. 9 is a sectional view showing a subsequent fabrication process forthe photovoltaic ultraviolet sensor of FIG. 1;

FIG. 10 is a sectional view showing a subsequent fabrication process forthe photovoltaic ultraviolet sensor of FIG. 1;

FIG. 11 is a view showing a sensing characteristic of the photovoltaicultraviolet sensor of FIG. 1;

FIG. 12 is a partially-sectional, perspective view showing aphotovoltaic ultraviolet sensor according to a second embodiment of thepresent invention;

FIG. 13 is a sectional view showing a fabrication process for thephotovoltaic ultraviolet sensor of FIG. 12;

FIG. 14 is a sectional view showing a subsequent fabrication process forthe photovoltaic ultraviolet sensor of FIG. 12;

FIG. 15 is a sectional view showing a subsequent fabrication process forthe photovoltaic ultraviolet sensor of FIG. 12;

FIG. 16 is a sectional view showing a subsequent fabrication process forthe photovoltaic ultraviolet sensor of FIG. 12;

FIG. 17 is a sectional view showing a subsequent fabrication process forthe photovoltaic ultraviolet sensor of FIG. 12;

FIG. 18 is a sectional view showing a subsequent fabrication process forthe photovoltaic ultraviolet sensor of FIG. 12;

FIG. 19 is a sectional view showing a subsequent fabrication process forthe photovoltaic ultraviolet sensor of FIG. 12;

FIG. 20 is a sectional view showing a fabrication process for aphotovoltaic ultraviolet sensor according to a third embodiment of thepresent invention;

FIG. 21 is a sectional view showing a subsequent fabrication process forthe photovoltaic ultraviolet sensor of FIG. 20;

FIG. 22 is a sectional view showing a subsequent fabrication process forthe photovoltaic ultraviolet sensor of FIG. 20;

FIG. 23 is a sectional view showing a subsequent fabrication process forthe photovoltaic ultraviolet sensor of FIG. 20;

FIG. 24 is a sectional view showing a subsequent fabrication process forthe photovoltaic ultraviolet sensor of FIG. 20;

FIG. 25 is a sectional view showing a subsequent fabrication process forthe photovoltaic ultraviolet sensor of FIG. 20;

FIG. 26 is a sectional view showing a fabrication process for aphotovoltaic ultraviolet sensor according to a fourth embodiment of thepresent invention;

FIG. 27 is a sectional view showing a subsequent fabrication process forthe photovoltaic ultraviolet sensor of FIG. 26;

FIG. 28 is a sectional view showing a subsequent fabrication process forthe photovoltaic ultraviolet sensor of FIG. 26;

FIG. 29 is a sectional view showing a subsequent fabrication process forthe photovoltaic ultraviolet sensor of FIG. 26;

FIG. 30 is a sectional view showing a subsequent fabrication process forthe photovoltaic ultraviolet sensor of FIG. 26;

FIG. 31 is a sectional view showing a subsequent fabrication process forthe photovoltaic ultraviolet sensor of FIG. 26;

FIG. 32 is a sectional view showing a fabrication process for aphotovoltaic ultraviolet sensor according to a fifth embodiment of thepresent invention;

FIG. 33 is a sectional view showing a subsequent fabrication process forthe photovoltaic ultraviolet sensor of FIG. 32;

FIG. 34 is a sectional view showing a subsequent fabrication process forthe photovoltaic ultraviolet sensor of FIG. 32;

FIG. 35 is a sectional view showing a subsequent fabrication process forthe photovoltaic ultraviolet sensor of FIG. 32;

FIG. 36 is a sectional view showing a subsequent fabrication process forthe photovoltaic ultraviolet sensor of FIG. 32; and

FIG. 37 is a sectional view showing a subsequent fabrication process forthe photovoltaic ultraviolet sensor of FIG. 32.

DESCRIPTION OF PREFERRED EMBODIMENTS

With reference to FIG. 1, a photovoltaic ultraviolet sensor according toa first embodiment of the present invention comprises a zinc oxidesingle crystal substrate 11 as a main component. The zinc oxide singlecrystal substrate 11 is one obtainable by cutting a zinc oxide singlecrystal 1 as shown in FIG. 2 along with a plane perpendicular to its caxis 3 so as to form a plurality of wafers 2, followed by carrying out aheat treatment for one of the wafers 2 under an oxygen-containingatmosphere within a container made of high-purity zinc oxide. Thus, theheat-treated wafer 2 can be obtained, which includes a plurality of zincoxide single crystal substrates 11. After a plurality of photovoltaicultraviolet sensors are formed on the heat-treated wafer 2, the wafer 2is diced so that the plurality of photovoltaic ultraviolet sensors, i.e.the zinc oxide single crystal substrates 11 are obtained together withpredetermined components described hereinafter in detail.

As apparent from the above description, the zinc oxide single crystalsubstrate 11 has two surfaces which are +c face and −c face of the zincoxide single crystal. In detail, as shown in FIG. 3, the −c face 5 ofthe zinc oxide single crystal is a face defined by four oxygen atoms andis also referred to as “O-face,” while the +c face 6 of the zinc oxidesingle crystal is a face defined by four zinc atoms and is also referredto as “Zn-face.” Another face 4 perpendicular to the −c face 5 and +cface 6 is referred to as “a” face. In addition, the zinc oxide singlecrystal as shown in FIG. 2 is formed in accordance with the hydrothermalsynthesis method using LiOH or KOH as a mineralizer. The zinc oxidesingle crystal may be formed in accordance with another method, forexample, vapor deposition method, flux method, scorification method,molecular beam epitaxy (MBE) method, vacuum deposition method, metalorganic chemical vapor deposition (MOCVD) method, or spattering method.

Turning back to FIG. 1, the photovoltaic ultraviolet sensor of thepresent embodiment further comprises an ultraviolet receiver formed onthe +c face of the zinc oxide single crystal substrate 11. The ultraviolet receiver of the present embodiment is a Schottky electrode 12formed directly on the +c face of the zinc oxide single crystalsubstrate 11, for example, under an oxygen atmosphere. The Schottkyelectrode 12 serves as an ultraviolet receiver surface and, whenreceiving ultraviolet rays, produces a voltage in cooperation with thezinc oxide single crystal substrate 11. The Schottky electrode 12 maycomprise one or more layers, each of which is made of Pt, Ru, Pd, Au,Ni, Ir, Os, Re, Rh, Te or W. The Schottky electrode 12 of the presentembodiment has a predetermined thickness such that the Schottkyelectrode 12 is ultraviolet-permeable. Instead, the Schottky electrode12 may have another thickness thicker than the predetermined thickness,provided that the Schottky electrode 12 has a special shape such as acomb-like such that ultraviolet rays are allowed to reach the Schottkyjunction between the Schottky electrode 12 and the +c face 6 of the zincoxide single crystal.

As shown in FIG. 1, the photovoltaic ultraviolet sensor of the presentembodiment further comprises a passivation film 13, an additionalelectrode 14, an antireflection film 15, an AZO(Al-doped Zinc Oxide)thin film 16 and an ohmic electrode 17.

The passivation film 13 is made of one or more insulator materials andcovers a peripheral region on the +c face of the zinc oxide singlecrystal substrate 11 and a peripheral part of the Schottky electrode 12.The passivation film 13 may comprise at least a layer made of Al₂O₃,SiO₂, SiNO, SiN, AlN, SIALON(silicon aluminum oxynitride), ZnS or ZnO.

The additional electrode 14 is electrically connected to the Schottkyelectrode 12 and is formed on the passivation film 13 so that theSchottky electrode 12 is electrically accessible from the outsidethrough the additional electrode 14. The additional electrode 14 maycomprise one or more layers, each of which is made of Pt, Ru, Pd, Au, orNi.

The antireflection film 15 is formed on a receiving portion of theultraviolet receiver, i.e. a center region of the Schottky electrode 12as shown in FIG. 1. The antireflection film 15 has a thickness of 1 to200 nm and is ultraviolet-permeable. The antireflection film comprisesone or more layers, each of which is made of Al₂O₃, SiO₂, SiNO, SiN, ZnSor ZnO.

The AZO thin film 16 is formed on the −c face of the zinc oxide singlecrystal substrate 11, and the ohmic electrode 17 is formed on the AZOthin film 16. The ohmic electrode 17 may comprise one or more layers,each of which is made of Al, Cr, Zn, Ti, Ru, Pd, Pt, Ni, In, Au, Cu orW.

Now, an explanation will be made about fabrication processes for theexemplary photovoltaic ultraviolet sensor according to the presentembodiment, with reference to FIGS. 4 to 10.

First, the heat-treated wafer 2 including the zinc oxide single crystalsubstrates 11 is prepared in a manner as described above. However, FIG.4 shows only one zinc oxide single crystal substrate 11, for the sake ofclarity. For the same reason, FIGS. 5 to 10 are illustrated inconnection with only one zinc oxide single crystal substrate 11. Theillustrated zinc oxide single crystal substrate 11 has +c face as anupper surface and −c face as a lower surface.

Next, as shown in FIG. 5, the Schottky electrode 12 is formed, forexample, by forming a first resist pattern on the upper surface of thezinc oxide single crystal substrate 11 by means of the photolithographytechnique, followed by forming a Pt thin film of 3 nm by means of thesputtering method, further followed by lifting off the first resistpattern together with the Pt thin film formed thereon.

Next, as shown in FIG. 6, the passivation film 13 is formed, forexample, by forming a second resist pattern only covering the centerarea of the Schottky electrode 12, followed by forming a SiO₂ thin filmof 200 nm by means of the sputtering method, further followed by liftingoff the second resist pattern together with the SiO₂ thin film formedthereon. After the formation of the passivation film 13, the thusobtained intermediate product is subjected to a heat treatment process.

Next, as shown in FIG. 7, the additional electrode 14 is formed, forexample, by forming a third resist pattern, followed by forming a Ptthin film of 300 nm by means of the sputtering method, further followedby lifting off the third resist pattern together with the Pt thin filmformed thereon, wherein the third resist pattern has two parts, one ofwhich is a center pattern smaller than the second resist pattern anddeposited on the center area of the Schottky electrode 12, the other oneis a peripheral pattern having a first width and positioned on theperipheral portion of the passivation film 13.

Next, as shown in FIG. 8, the antireflection film 15 is formed, forexample, by forming a fourth resist pattern on the peripheral portion ofthe thus obtained intermediate product, followed by forming a SiO₂ thinfilm of 60 nm by means of the sputtering method, further followed bylifting off the fourth resist pattern together with the SiO₂ thin filmformed thereon, wherein the fourth resist pattern has a second widthwider than the first width of the third resist pattern. After theformation of the antireflection film 15, the thus obtained intermediateproduct is subjected to a heat treatment process.

Next, as shown in FIG. 9, the AZO thin film 16 is formed on the lowersurface of the zinc oxide single crystal substrate 11, for example, bythe sputtering method, wherein the exemplary AZO thin film 16 has athickness of 100 nm.

Next, as shown in FIG. 10, an Al thin film of 300 nm is formed as theohmic electrode 17 on the AZO thin film 16, for example, by thesputtering method. After that, as mentioned above, the wafer is dicedinto the ultraviolet sensor chips, each of which has a size of 1 mm(H)×1mm(W)×0.3 mm(T).

The thus obtained ultraviolet sensor according to the present embodimenthas an ultraviolet sensing characteristic shown in FIG. 11. The measuredbandwidth is from 250 nm to 600 nm, while the response bandwidth is from250 nm to 380 nm. As understood from the illustrated characteristic, theultraviolet sensor of the present embodiment is usable for sensing UV-A(ultraviolet A) of 320 to 400 nm, UV-B (ultraviolet B) of 280 to 320 nm,and UV-C (ultraviolet C) of 280 nm or smaller. The illustratedcharacteristic further shows that the exemplary ultraviolet sensor has asensitivity peak on approximately 350 nm but does not have anysensitivity to the visible rays. In addition, the exemplary ultravioletsensor has a relatively fast response of several microseconds.

Next explanation will be made about a photovoltaic ultraviolet sensoraccording to a second embodiment of the present invention, withreference to FIG. 12.

The ultraviolet sensor of the present embodiment is a modification ofthe ultraviolet sensor of the first embodiment. In this connection, likenumerals are used to denote like elements in FIGS. 1 and 12. ComparingFIGS. 1 and 12, the ultraviolet sensor of the present embodiment has astructure similar to that of the first embodiment except that thepassivation film 13 is formed under the peripheral part of the Schottkyelectrode 12.

The ultraviolet sensor of the present embodiment can be obtained in thefollowing manner described hereinafter with reference to FIGS. 13 to 19.

First, the heat-treated wafer including the zinc oxide single crystalsubstrates 11 is prepared as shown in FIG. 13. The illustrated zincoxide single crystal substrate 11 has +c face as an upper surface and −cface as a lower surface.

Next, as shown in FIG. 14, the passivation film 13 is formed, forexample, by forming an Al₂O₃ thin film of 200 nm by means of thesputtering method on the upper surface of the zinc oxide single crystalsubstrate 11, followed by forming a first resist pattern on theperipheral portion of the Al₂O₃ thin film, further followed by etchingthe Al₂O₃ thin film by the use of the first resist pattern as a mask.After the formation of the passivation film 13, the thus obtainedintermediate product is subjected to a heat treatment process.

Next, as shown in FIG. 15, the Schottky electrode 12 is formed, forexample, by forming a Pt thin film of 3 nm on a region including anexposed portion of the upper surface of the zinc oxide single crystalsubstrate 11 by means of the sputtering method and photolithographytechniques.

Next, as shown in FIG. 16, the additional electrode 14 is formed, forexample, by forming a second resist pattern, followed by forming a Ptthin film of 300 nm by means of the sputtering method, further followedby lifting off the second resist pattern together with the Pt thin filmformed thereon, wherein the second resist pattern has two parts, one ofwhich is a center pattern deposited on the center area of the Schottkyelectrode 12, the other one is a peripheral pattern having a first widthand positioned on the peripheral portion of the passivation film 13.

Next, as shown in FIG. 17, the antireflection film 15 is formed, forexample, by forming a third resist pattern on the peripheral portion ofthe thus obtained intermediate product, followed by forming a SiO₂ thinfilm of 60 nm by means of the sputtering method, further followed bylifting off the third resist pattern together with the SiO₂ thin filmformed thereon, wherein the third resist pattern has a second widthwider than the first width of the second resist pattern. After theformation of the antireflection film 15, the thus obtained intermediateproduct is subjected to a heat treatment process.

Next, as shown in FIG. 18, the AZO thin film 16 is formed on the lowersurface of the zinc oxide single crystal substrate 11, for example, bythe sputtering method, wherein the exemplary AZO thin film 16 has athickness of 100 nm.

Next, as shown in FIG. 19, an Al thin film of 300 nm is formed as theohmic electrode 17 on the AZO thin film 16, for example, by thesputtering method. After that, as mentioned above, the wafer is dicedinto the ultraviolet sensor chips, each of which has a size of 1 mm(H)×1 mm (W)×0.3 mm (T).

Next explanation will be made about a photovoltaic ultraviolet sensoraccording to a third embodiment of the present invention, with referenceto FIGS. 20 to 25. The ultraviolet sensor of the present embodiment is amodification of the ultraviolet sensor of the first embodiment. In thisconnection, like numerals are used to denote like elements in FIGS. 1and 20 to 25.

Comparing FIGS. 1 and 25, the ultraviolet sensor of the presentembodiment has a structure similar to that of the first embodimentexcept for two points. One point is that an adjustment thin film 18 isinterposed between the zinc oxide single crystal substrate 11 and theSchottky electrode 12. The other point is that the ultraviolet sensor ofthe present embodiment neither has the passivation film 13 nor theantireflection film 15. However, the passivation film 13 and/or theantireflection film 15 may be included in a manner similar to the firstor the second embodiment.

The adjustment thin film 18 is a thin film whose resistivity, conductiontype, i.e. n- or p- type, and/or band gap are adjusted by adding atleast one kind additive into a base material. The adjustment thin film18 of the present embodiment comprises a zinc oxide based thin film. Inaddition, the adjustment thin film 18 of the zinc oxide based thin filmmay comprise at least one layer which is a chemical compound selectedfrom the group consisting of Ca, Mg, S, Al, Cd, Se, Ga, N, Cu and Te.

The ultraviolet sensor of the present embodiment can be obtained in thefollowing manner described hereinafter with reference to FIGS. 20 to 25.

First, the heat-treated wafer including the zinc oxide single crystalsubstrates 11 is prepared as shown in FIG. 20. The illustrated zincoxide single crystal substrate 11 has +c face as an upper surface and −cface as a lower surface.

Next, as shown in FIG. 21, the adjustment thin film 18 is formed overthe +c face of the zinc oxide single crystal substrate 11, for example,by the MOCVD method, wherein the adjustment thin film 18 is an epitaxiallayered zinc oxide based thin film into which Al is added as anadditive. The exemplary adjustment thin film 18 has resistivity of 100to 500 Ωcm and is of n-type.

Next, as shown in FIG. 22, the Schottky electrode 12 is formed on theadjustment thin film 18, for example, by forming a Pt thin film of 3 nmby means of the sputtering method. The Schottky electrode 12 and theadjustment thin film 18 constitute a Schottky barrier and, when theSchottky electrode 12 receives ultraviolet rays, produce a voltage.

Next, as shown in FIG. 23, the additional electrode 14 is formed, forexample, by forming a predetermined resist pattern, followed by forminga Pt thin film of 300 nm by means of the sputtering method, furtherfollowed by lifting off the predetermined resist pattern together withthe Pt thin film formed thereon.

Next, as shown in FIG. 24, the AZO thin film 16 is formed on the −c faceof the zinc oxide single crystal substrate 11, for example, by thesputtering method, wherein the exemplary AZO thin film 16 has athickness of 100 nm.

Next, as shown in FIG. 25, an Al thin film of 300 nm is formed as theohmic electrode 17 on the AZO thin film 16, for example, by thesputtering method. After that, as mentioned above, the wafer is dicedinto the ultraviolet sensor chips, each of which has a size of 1 mm(H)×1 mm (W)×0.3 mm (T).

Next explanation will be made about a photovoltaic ultraviolet sensoraccording to a fourth embodiment of the present invention, withreference to FIGS. 26 to 31. The photovoltaic ultraviolet sensoraccording to the present embodiment has not a Schottky junction but ap-n junction, different from the first to the third embodiments.However, because there are similar elements, for example, in FIGS. 1 and26 to 31, like numerals are used to denote like elements.

The zinc oxide single crystal substrate 11 of the present embodiment isof n-type. On the zinc oxide single crystal substrate 11, a p-type thinfilm 19 is formed. The p-type thin film 19 may comprise a p-type zincoxide thin film, a p-type zinc oxide based thin film, or a p-typenitride thin film such as p-type GaN thin film. In addition, the p-typethin film 19 of the zinc oxide based thin film may comprise at least onelayer which is a chemical compound selected from the group consisting ofCa, Mg, S, Al, Cd, Se, Ga, N, Cu and Te. On the other hand, the p-typethin film 19 of the nitride thin film may comprise at least one layerwhich is a chemical compound selected from the group consisting of Ga,Al and In.

The ultraviolet sensor of the present embodiment can be obtained in thefollowing manner described hereinafter with reference to FIGS. 26 to 31.

First, the heat-treated wafer including the n-type zinc oxide singlecrystal 10 substrates 11 is prepared as shown in FIG. 26. Theillustrated zinc oxide single crystal substrate 11 has +c face as anupper surface and −c face as a lower surface.

Next, as shown in FIG. 27, the p-type thin film 19 is formed directly onthe +c face of the zinc oxide single crystal substrate 11, for example,by the MOCVD method, wherein the p-type thin film 19 is an epitaxiallayered zinc oxide thin film or a GaN thin film.

Next, as shown in FIG. 28, the passivation film 13 is formed, forexample, by forming an Al₂O₃ thin film of 200 nm by means of thesputtering method over the upper surface of the zinc oxide singlecrystal substrate 11 and the p-type thin film 19, followed by forming afirst resist pattern on the Al₂O₃ thin film, further followed by etchingthe Al₂O₃ thin film by the use of the first resist pattern as a mask.The etching process provides the passivation film 13 with a centeredlarge aperture and a contact hole which has an annular shape. After theformation of the passivation film 13, the thus obtained intermediateproduct is subjected to a heat treatment process.

Next, as shown in FIG. 29, a first ohmic electrode 54 is formed, forexample, by forming a second resist pattern, followed by forming a Nithin film of 100 nm by means of the sputtering method, further followedby forming an Au thin film of 200 nm by means of the sputtering method,further followed by lifting off the predetermined resist patterntogether with the Ni thin film and the Au thin film formed thereon.

Next, as shown in FIG. 30, the AZO thin film 16 is formed on the −c faceof the zinc oxide single crystal substrate 11, for example, by thesputtering method, wherein the exemplary AZO thin film 16 has athickness of 100 nm.

Next, as shown in FIG. 31, an Al thin film of 300 nm is formed as asecond ohmic electrode 17 on the AZO thin film 16, for example, by thesputtering method. After that, as mentioned above, the wafer is dicedinto the ultraviolet sensor chips, each of which has a size of 1 mm(H)×1 mm (W)×0.3 mm (T).

Although the ultraviolet sensor of the present embodiment does not havean antireflection film, an antireflection film may be formed in a mannersimilar to the first or the second embodiment.

Next explanation will be made about a photovoltaic ultraviolet sensoraccording to a fifth embodiment of the present invention, with referenceto FIGS. 32 to 37. The ultraviolet sensor of the present embodiment is amodification of the ultraviolet sensor of the third embodiment. In thisconnection, like numerals are used to denote like elements in FIGS. 20to 25 and 32 to 37. The ultraviolet sensor of the present embodiment hasa structure similar to that of the third embodiment except that theadjustment thin film 21 is made not of a zinc oxide based thin film butof a nitride thin film interposed between the zinc oxide single crystalsubstrate 11 and the Schottky electrode 12. The adjustment thin-film 21of the nitride thin film may comprise at least one layer which is achemical compound selected from the group consisting of Ga, Al and In.

The ultraviolet sensor of the present embodiment can be obtained in thefollowing manner described hereinafter with reference to FIGS. 32 to 37.

First, the heat-treated wafer including the zinc oxide single crystalsubstrates 11 is prepared as shown in FIG. 32. The illustrated zincoxide single crystal substrate 11 has +c face as an upper surface and −cface as a lower surface.

Next, as shown in FIG. 33, the adjustment thin film 21 is formed overthe +c face of the zinc oxide single crystal substrate 11, for example,by the MOCVD method, wherein the adjustment thin film 21 is an epitaxiallayered n-type GaN. The exemplary adjustment thin film 21 has athickness of 1 μm.

Next, as shown in FIG. 34, the Schottky electrode 12 is formed on theadjustment thin film 21, for example, by forming a Pt thin film of 3 nmby means of the sputtering method. The Schottky electrode 12 and theadjustment thin film 21 constitute a Schottky barrier and, when theSchottky electrode 12 receives ultraviolet rays, produce a voltage.

Next, as shown in FIG. 35, the additional electrode 14 is formed, forexample, by forming a predetermined resist pattern, followed by forminga Pt thin film of 300 nm by means of the sputtering method, furtherfollowed by lifting off the predetermined resist pattern together withthe Pt thin film formed thereon.

Next, as shown in FIG. 36, the AZO thin film 16 is formed on the −c faceof the zinc oxide single crystal substrate 11, for example, by thesputtering method, wherein the exemplary AZO thin film 16 has athickness of 100 nm.

Next, as shown in FIG. 37, an Al thin film of 300 nm is formed as theohmic electrode 17 on the AZO thin film 16, for example, by thesputtering method. After that, as mentioned above, the wafer is dicedinto the ultraviolet sensor chips, each -of which has a size of 1 mm(H)×1 mm (W)×0.3 mm (T).

Although the ultraviolet sensor of the present embodiment neither has apassivation film nor an antireflection film, a passivation film and/oran antireflection film may be formed in a manner similar to the first orthe second embodiment.

The preferred embodiments of the present invention will be betterunderstood by those skilled in the art by reference to the abovedescription and figures. The description and preferred embodiments ofthis invention illustrated in the figures are not to intend to beexhaustive or to limit the invention to the precise form disclosed. Theyare chosen to describe or to best explain the principles of theinvention and its applicable and practical use to thereby enable othersskilled in the art to best utilize the invention.

While there has been described what is believed to be the preferredembodiment of the invention, those skilled in the art will recognizethat other and further modifications may be made thereto withoutdeparting from the sprit of the invention, and it is intended to claimall such embodiments that fall within the true scope of the invention.

1. A photovoltaic ultraviolet sensor comprising: a zinc oxide singlecrystal having a +c face; and an ultraviolet receiver which is formed onthe +c face of the zinc oxide single crystal and, when receivingultraviolet rays, produces a voltage solely or in cooperation with thezinc oxide single crystal.
 2. The photovoltaic ultraviolet sensoraccording to claim 1, wherein the ultraviolet receiver comprises aSchottky electrode formed directly on the +c face of the zinc oxidesingle crystal.
 3. The photovoltaic ultraviolet sensor according toclaim 1, wherein the ultraviolet receiver comprises an adjustment thinfilm directly formed on the +c face of the zinc oxide single crystal anda Schottky electrode formed on the adjustment thin film.
 4. Thephotovoltaic ultraviolet sensor according to claim 3, wherein theadjustment thin film comprises a zinc oxide based thin film.
 5. Thephotovoltaic ultraviolet sensor according to claim 4, wherein the zincoxide based thin film comprises at least one layer which is a chemicalcompound selected from the group consisting of Ca, Mg, S, Al, Cd, Se,Ga, N, Cu and Te.
 6. The photovoltaic ultraviolet sensor according toclaim 3, wherein the adjustment thin film comprises a nitride thin film.7. The photovoltaic ultraviolet sensor according to claim 6, wherein thenitride thin film comprises at least one layer which is a chemicalcompound selected from the group consisting of Ga, Al and In.
 8. Thephotovoltaic ultraviolet sensor according to claim 2, wherein theSchottky electrode is ultraviolet-permeable.
 9. The photovoltaicultraviolet sensor according to claim 2, wherein the Schottky electrodecomprises one or more layers, each of which is made of Pt, Ru, Pd, Au,Ni, Ir, Os, Re, Rh, Te or W.
 10. The photovoltaic ultraviolet sensoraccording to claim 1, wherein the zinc oxide single crystal is ofn-type, and the ultraviolet receiver comprises a p-type thin film whichis formed directly on the +c face of the zinc oxide single crystal andconstitutes a p-n junction in cooperation with the zinc oxide singlecrystal.
 11. The photovoltaic ultraviolet sensor according to claim 10,wherein the p-type thin film comprises a zinc oxide thin film.
 12. Thephotovoltaic ultraviolet sensor according to claim 10, wherein thep-type thin film comprises a zinc oxide based thin film.
 13. Thephotovoltaic ultraviolet sensor according to claim 12, wherein the zincoxide based thin film comprises at least one layer which is a chemicalcompound selected from the group consisting of Ca, Mg, S, Al, Cd, Se,Ga, N, Cu and Te.
 14. The photovoltaic ultraviolet sensor according toclaim 10, wherein the p-type thin film comprises a nitride thin film.15. The photovoltaic ultraviolet sensor according to claim 14, whereinthe nitride thin film comprises at least one layer which is a chemicalcompound selected from the group consisting of Ga, Al and In.
 16. Thephotovoltaic ultraviolet sensor according to claim 1, further comprisingat least one ohmic electrode, wherein the zinc oxide single crystal hasa −c face opposite to the +c face, and the at least one ohmic electrodeis formed on the −c face and comprises one or more layers, each of whichis made of Al, Cr, Zn, Ti, Ru, Pd, Pt, Ni, In, Au, Cu or W.
 17. Thephotovoltaic ultraviolet sensor according to claim 1, further comprisingan antireflection film, wherein the ultraviolet receiver has a receivingportion, and the antireflection film covers the receiving portion. 18.The photovoltaic ultraviolet sensor according to claim 17, wherein theantireflection film has a thickness of 1 to 200 nm.
 19. The photovoltaicultraviolet sensor according to claim 17, wherein the antireflectionfilm is ultraviolet-permeable.
 20. The photovoltaic ultraviolet sensoraccording to claim 17, wherein the antireflection film comprises one ormore layers, each of which is made of Al₂O₃, SiO₂, SiNO, SiN, ZnS orZnO.
 21. The photovoltaic ultraviolet sensor according to claim 1,further comprising a passivation film, wherein the zinc oxide singlecrystal has a first peripheral region on the +c face of the zinc oxidesingle crystal, the ultraviolet receiver has a second peripheral partpositioned on the first peripheral region, and the passivation filmcovers at least one of the first peripheral region and the secondperipheral part.
 22. The photovoltaic ultraviolet sensor according toclaim 21, wherein the passivation film comprises at least a layer madeof Al₂O₃, SiO₂, SiNO, SiN, AlN, SIALON, ZnS or ZnO.
 23. The photovoltaicultraviolet sensor according to claim 1, wherein the zinc oxide singlecrystal is one obtainable by carrying out a heat treatment for a basematerial of zinc oxide single crystal under an oxygen-containingatmosphere within a container made of zinc oxide.